MXPA97005458A

MXPA97005458A - Process for the formation of pelotillas from amy polyester

Info

Publication number: MXPA97005458A
Application number: MXPA/A/1997/005458A
Authority: MX
Inventors: M Stouffer Jan; Neal Blanchard Elwood; Wayne Leffew Kenneth
Original assignee: E I Du Pont De Nemours And Company
Priority date: 1995-01-20
Filing date: 1997-07-18
Publication date: 1998-07-03

Abstract

The present invention relates to a process for the formation of crystalline polyester particles, robust, uniform, of low molecular weight. Such particles are useful for the formation of higher molecular weight polyesters, for example, by solid state polymerization. The non-molten polyester pellets, either conventionally formed, essentially amorphous, at ambient temperatures or, alternatively, the essentially amorphous molten polyester droplets, can be crystallized by rapidly attaching the droplets or pellets to a thermal shock according to the process of the present invention

Description

PROCESS FOR THE FORMATION OF PELOTILLAS FROM AMORFO POLYESTER FIELD OF THE INVENTION This invention relates to a process for the formation of crystalline, uniform, low molecular weight polyester particles. Such particles are useful as a raw material for the production of high molecular weight polyester.

BACKGROUND IS OF THE INVENTION Various methods and apparatuses for the formation of a particulate polymer are known. For example, polymers have been formed into solidified strands, tapes or sheets, which have then been broken into particles. Fracturing or granulating a sheet, for example, into particles can be achieved by various methods including ball milling. Such methods of particle formation, however, can result in particles that are not of uniform size and shape. In addition, such methods can generate an undesirable amount of fine particles, which REF: 25185 make the handling of particles and processing difficult. It is also known to form polyester particles primarily by forming polyester drops from a "melt" of the polyester, and subsequently solidifying the droplets of particles or pellets. The formation of pellets or particles is an example of one such method. For example, Chang et al. US Patent No. 5,340,509, describe a process for the formation of pellets of crystalline polymers with ultra high melt flow, which are polyolefin homopolymers, copolymers, or mixtures thereof. The process of Chang et al. Uses a means for the formation of droplets, a pelletizer which comprises an outer container with holes. The outer container rotates around an internal container to allow a uniform amount of the molten polymer to emerge as droplets. The droplets are collected on a conveyor, which cools the droplets for a sufficient time to solidify the droplets. In addition to the tabletting methods, the formation of particulate polymers by polymer droplets has been achieved in a variety of other ways. For example, U.S. Patent No. 4,340,550 to Ho discloses the preparation of free-flowing pellets of poly (polyethylene terephthalate) oligomer by quenching the aotites of the molten oligomer in water. The molten oligomer is fed to a droplet forming medium having a plate with holes, with multiple orifices. Under pressure, the molten oligomer * flows through the orifices and outwards into an inert gas. The molten oligomer dissociates into droplets at a distance from the plate under the force of surface tension. The molten droplets are then turned off in a water tank. The oligomer pellets are slightly flattened, approximately 0.3 to 2.0 mm thick and approximately 0.8 to 4.0 mm in circular diameter. Rimehart, US Pat. No. 4,165,420, discloses the formation of particles by the use of a spray freezer which forms particles from the low viscosity molten polymer. The molten polymer is transported to the rotating drum of a centrifugal atomizing device. This device produces small spherical droplets which coagulate or freeze, in an inert gas, in the form of spherical pellets having an average particle size of 100 to 250 microns, depending on the speed of rotation of the drum. Uniform, crystalline, low molecular weight polyester particles in size ranges suitable for mass handling (eg, from about 2 mm to 6 mm) are difficult to produce using traditional methods and apparatus for various reasons. A low molecular weight polyester, also referred to as an oligomer or prepolymer, when in a molten state, may have relatively low viscosity. Such low viscosity can cause difficulties in the formation of droplets of uniform size and shape, especially by conventional means due to the resulting low pressures. Low molecular weight polyester particles, as produced by conventional methods, have the disadvantage that they can not be in a more conductive form to solid state polymerization (SSP), especially in the absence of an annealing step that consumes weather. Solid state polymerization is used in the industry to obtain polycondensation polymers of high quality grade, of very high molecular weight. Such solid state polymerization typically involves heating a "prepolymer" which is a medium molecular weight polymer, in the form of comminutes. This polymer was heated to a temperature above its vitreous transition temperature (Tg) but by its melting point (TJ) In comparison, relatively low molecular weight particles, as raw materials, can be disadvantaged due to the difficulty in the formation of such particles, and due to the fragile nature of the particles formed.As polymerization reaction rates increase with temperature, the optimum temperature for polymerization in the solid state is usually so close to the melting point As possible, in order to reduce stickiness or adhesion together at such a high temperature, polyester particles produced by conventional particulate formation methods and conventional apparatuses typically require conditioning prior to solid state polymerization. Conditioning may involve annealing at clear temperatures Highly (for example, from 150 ° C to 210 ° C for polyethylene terephthalate) and for prolonged periods of time (for example, from about 0.5 to 8 hours). Such conditioning increases the level of crystallinity of the particles. Typically, the pellets are initially subjected to a certain amount of annealing under high turbulence and agitation, in order to achieve uniform annealing without joint adhesion. If such particles are not adequately conditioned prior to polymerization in the solid state, processing problems can result. For example, these may tend to adhere together during the SSP, resulting in an inability to discharge the particles from the SSP reactor, which can even result in a shutdown of the reactor. As mentioned above, the polyester particles or pellets formed by conventional methods may be unduly non-uniform, malformed and / or characterized by high levels of fine particles. Such malformed and non-uniform pellets can be undesirable because they can form sources in pellet feed hoppers. In addition, significant quantities of malformed pellets can alter the apparent density of the raw material in the form of pellets for a polymerization process, which can result in feeding problems in the extrusion lines. This can also result in empty spaces in the final product. Since the reaction rate is to a certain degree dependent on particle size, non-uniform pellets can result in non-uniform molecular weight in the polymerization product. In view of the foregoing, there is a need for an improved process of polyester particle formation. In order to be useful as raw materials for the polymerization processes, such particles preferably must have sufficient structural integrity to make them suitable for transport to such processes. The particles should preferably have relatively uniform size and shape, in order to facilitate handling and ensure uniform polymerization within each particle. For solid state polymerization, the particles should preferably be sufficiently robust to withstand the high temperatures during solid state polymerization, without agglomeration. It could be even more desirable if the particles could withstand higher temperatures than those more typical of solid state polymerization. It might be desirable if robust or crystalline particles could be obtained more efficiently and easily than is currently the norm. Accordingly, it could be advantageous if expensive and cumbersome steps for conditioning polyester particles prior to solid state polymerizations or other polymerizations could be reduced or eliminated. It may be desirable that such particles could have a variety of uses, including, not only serving as a prepolymer or raw material for polymerization in the solid state, but optionally or additionally as a feedstock for, among other examples, injection molding. , bottle making and extrusion processes.

BRIEF DESCRIPTION OF THE INVENTION The present invention is directed to a process for preparing a polyester pellet in crystalline form. The pellets formed by the present invention are in the size range of between about 500 microns and 2 cm in average diameter. The process can be carried out either by crystallization of molten droplets of an amorphous melt or, alternatively, by crystallization of pellets or particles of an amorphous solid. In any case, the process can be described as a heat shock treatment. A polyester particle is crystallized by subjecting the particle to a rapid change in its ambient temperature, so that the particle, or its apparent average temperature, is brought sufficiently quickly to a temperature within a certain temperature range or zone. Without wishing to be bound by theory, it is believed that this rapid change in particle temperature allows the particle to undergo crystallization, mainly within the desired temperature range, instead of suffering the particle an undesirable amount and / or type of crystallization before reach this temperature range. Preferably, a minimum amount of crystallization takes place before the particle reaches the desired temperature range or zone. This temperature zone extends through what is termed the "maximum crystallization rate", calculated. As mentioned above, the particle can be brought within that temperature range either from an amorphous melt or from an amorphous solid or glass, which is from any direction, to the calculated point of maximum crystallization rate. The calculated point of maximum crystallization rate (the calculated T) for polyesters is defined as the intermediate point between the vitreous transition temperature (T3) of the polyester and the melting point (T- of the polyester.) Thus, T -is equal to 11 + 0.5 (T-Ta) .The same value for T can be obtained by the equation T- = 0.5 (Tm + T3) .This calculated Tr is a reasonably accurate approximation of an experimentally determined Tr or Measurement of each polymer Unless otherwise stated, T- in the present will refer to the T-theoretical or calculated, as defined herein, rather than to the experimentally-measured T. performed with the particle, the present invention can be defined as a process for the crystallization of low molecular weight polyester pellets, whose process comprises: (1) heating the solid pellets (crystalline) of a polyester oligomer, which has a degree of polymerization (DP) from 2 to 40 and a glass transition temperature (Ta) above 25 ° C, from an initial temperature T-Jf where Tg + 20 ° C, so that the apparent average temperature of the pellets is carried, within 15 seconds, at a temperature within a range extending from Tm? n to Tmax, where T ^. = Tc - 0.5 (TC-Tg) and Tm3x = Tc + 0.5 (Tm - Tc) and, furthermore, after reaching said temperature, the pellet is kept within the interval for at least 3 seconds; or (2) cooling the molten droplets of a polyester oligomer having the DP and T, as described above, from an initial temperature T where T: is at least the melting point Tt of the polyester oligomer , so that the apparent average temperature of the droplets or pellets of crystallization is brought, within 15 seconds, to a temperature within a range extending from T ~ Lr? to T ^ ax, where Tm? n = T, - 0.5 (TC-Tg) and Tmax = Tc + 0.5 (Tm-Tr) and, furthermore, after reaching said temperature, the pellet is maintained within said interval by at least 3 seconds In the last definition of the invention, the temperatures T ^ ip and T ^ refer to the temperature of the pellets. The invention can also be defined in terms of the thermal environment to which the pellets are exposed, including the parameters of the process. Accordingly, when essentially crystalline polyester pellets are formed from an essentially amorphous melt, a process according to the present invention can be defined as comprising the following steps: (a) the formation of molten droplets of an oligomer of polyester at a temperature T where T: is at least the melting point Tm of the polyester oligomer, and wherein the polyester oligomer has a degree of polymerization (DP) of 2 to 40 and a glass transition temperature (Ta) above 25 ° C; b) the crystallization of said molten droplets by placing the droplets in contact, for at least 3 seconds, with a solid surface which is at a temperature within the range of T ~ 5X, as defined below, with which droplets or pellets of crystallization sustain a rapid change in temperature toward said temperature, and remain at a temperature within the range for a sufficient period of time; where T,? n = Tg + 10 ° C, Tmax = Tc + 0.5 (Tm - T :), and T: = Tg + 0.5 (T, - T-); except that, if the solid surface has a heat transfer coefficient (hs) that is below 1.5 joules / second cm2 ° C, then Tm? n of the solid surface may be between 0 ° C and (Tg + 10 ° C) ) with the proviso that the change in the average apparent temperature of the pellets remains above T Í- for at least 3 seconds after the pellets come into contact with the solid surface, and with the condition that the average temperature Apparent of the pellets reach Tpax within 15 seconds, after contact of the pellets with the sole surface. Alternatively, as indicated above, essentially crystalline polyester pellets can be formed by starting with an essentially amorphous pellet which has been previously made from a polymer melt. In this case, the invention comprises the following steps: (a) obtaining pellets of a solid, essentially amorphous polyester oligomer, having a degree of polymerization (DP) of 2 to 40, and a glass transition temperature (Tg) ) above 25 ° C; (b) thermal treatment of the pellets, from a temperature?, by contacting them with a gas for at least about 0.5 seconds; wherein Tmin is at least the melting point of the polyester cliomer. In the present process, the purpose of exposing the pellets to contact with a hot or cold surface is to rapidly bring the pellets within this certain temperature range for a certain minimum period of time. However, this can be accomplished under a variety of circumstances. For example, in cooling the pellets if the surface to which the pellets are exposed has a relatively low heat transfer coefficient, it may be necessary, in order that the pellets rapidly reach the desired temperature, that the surface is significantly cooler than the desired temperature. For example, it may be necessary for the surface that it be significantly cooler when the surface is a plastic such as Teflon (MR) poly (tetrafluoroethylene) than when the surface is steel. Polyester pellets made by the process of this invention have a variety of uses, but pellets are especially advantageous for use as a prepolymer raw material for polymerization in the solid state, to produce higher molecular weight polyesters. Accordingly, the present process can be especially advantageous as part of a complete process for the manufacture of a higher molecular weight polyester, for example, by solid state polymerization. The present invention can be advantageously integrated with an early step in the process for the preparation of a polyester oligomer having the specified DP and / or with a late step of the process for the polymerization of the oligomer, which has been formed into pellets of according to the present invention. The invention can be more fully understood by reference to the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plot of the heat flow versus temperature, produced by the measurement by differential scanning calorimeter (DSC) of a. sample of an essentially amorphous polyethylene terephthalate pellet prior to conventional annealing or for processing according to the present invention. Figure 2 is a plot of the heat flow versus temperature, produced by a differential scanning calorimeter measurement of a conventional, low molecular weight polyethylene terephthalate pellet sample, annealed at 200 ° C for 1 hour, the graph of which shows a prefusion endotherm.

Figure 3 is a graph of heat flow versus temperature, produced by a differential scanning calorimeter measurement of a sample of a low molecular weight poly (ethylene terephthalate) pellet., made by the process of this invention by thermal shock crystallization of essentially amorphous PET pellets at 550 ° C for 25 seconds, the graph of which does not show a distinctive prefusion endotherm. Figure 4 is a plot of the heat flux versus temperature produced by a differential scanning calorimeter measurement of a sample of a low molecular weight poly (ethylene terephthalate) pellet, by the process of this invention by thermal shock crystallization in which newly formed PET droplets, initially at a temperature above the melting temperature, are exposed to a heated surface at a temperature of 170 ° C for 28 seconds, whose graph does not have a different prefusion endotherm . Figure 5 is an X-ray diffraction pattern illustrative of a sample of a PET polymer made according to the process of the present invention.

Figure 6 is another X-ray diffraction pattern illustrative of a sample of a PET polymer made according to the process of the present invention. Figure 7 is an X-ray diffraction pattern illustrative of a sample of a PET polymer made according to a process of the present invention, whose pattern has been developed in two overlapping Gaussian peaks.

DETAILED DESCRIPTION OF THE INVENTION This invention is directed to an improved process for producing essentially crystalline particles of a low molecular weight polyester. In one embodiment of the present invention or process, a polyester oligomer is formed, at or above the melting temperature of the polyester, in essentially crystalline pellets. In a second embodiment of the present invention, an essentially amorphous, low molecular weight, non-molten (crystalline or solid) polyester, such as that made by conventional methods, is formed into essentially crystalline pellets.

Both modalities can be characterized as crystallization of the pellets by a "heat shock" method, although the latter modality may involve a greater temperature differential between the pellets and the environment, due to diffusion or slower thermal conduction. Whether starting with non-melted, essentially amorphous, conventionally formed polyester pellets, at about ambient temperatures, or molten, newly formed polyester droplets, crystallization involves holding the pellets to a thermal environment such that the pellets quickly reach a temperature within the zone or region of the calculated T, the temperature of the maximum crystallization rate for the particular polyester that is processed. The particles must remain within this zone or region for a sufficient period of time, in order for a sufficient amount of crystallization to occur within that zone. During the thermal shock imosposition, the pellets are exposed to a temperature in the range of about T-, ax to Tp;., For at least a period of about 3 seconds. As a result, the crystalline structure of the pellets can quickly reach the desired degree of crystallinity and, in some cases, achieve a superior or even, in some cases, unique crystal morphology. Another possible advantage of crystallization in this way is that the pellets or polyester particles obtained in this way can be more easily polymerized in the solid state, potentially avoiding the conditioning steps currently thought to be necessary, as with the polyester particles. conventionally produced which require hours of processing time to achieve the necessary crystallization state for the solid state polymerization. The use of the present process for the production of polyester pellets for polymerization in solid state offers significant advantages in terms of economy of time and money, in addition to any improvement in quality. The term "pellets" is understood herein as any discrete unit or mass of a given material, having any shape or configuration, irregular or regular, within a wide range of sizes. Although the term "pellets" may also have a narrower connotation, the term "pellets" is used herein to include particles and pellets in the broadest sense of the word. The preferred shapes and / or sizes for the particles are spherical particles with diameters of 0.05 mm to 0.3 mm, hemispherical particles with a maximum cross section of 0.1 mm to 0.6 mm, or straight circular cylinders with a diameter of 0.05 mm to 0.3 mm and a length of 0.1 cm to 0.6 cm. Preferably, since the pellets are produced most economically efficient, the pellets could preferably be produced and collected together in commercial quantities of more than 10 kg., more preferably of more than 30 kg. The pellets can be used in the same plant shortly after being processed, stored for later use, or packed for transport, all in commercial quantities. By the term "polymer" is meant a compound or mixture of compounds consisting essentially of or comprising at least 90 percent, preferably at least 95 percent, and more preferably at least 99 percent by weight of repeating structural units called monomers. The term "polymer" is understood to include the prepolymer or the oligomer, i.e., a polymer having a degree of polymerization (DP) of at least 2 or 3. "Low molecular weight polymer" means a polymer having a degree of of polymerization in the range of about 2 to about 40, preferably 5 to 35. By "molten polymer" is meant a polymer at a temperature at or above its melting point; likewise "molten droplet" means any discrete unit, or portion, of an "anido which is a precursor polymer pellet, whether it is effectively formed in droplet or not, which is at a temperature of or above Polymer melting point "Pellet diameter" means the largest transverse dimension of a given pellet The "average diameter" or "average pellet size" means the largest, average, transverse dimension of a sample. representative of the pellets that are processed according to the present invention The melting point, T-, of a polymer is preferably determined as the maximum of the melting endotherm on the first heat, as measured by Differential Scanning Calorimetry (DSC) By a prefusion endotherm is meant an endothermic peak in the DSC due to a melting endotherm at a lower temperature than (before) the "main" melting endotherm. By a "different prefusion endotherm" is meant that the melting occurs over a temperature range of 60 ° C or less, preferably less than 40 ° C. By "not" having a prefusion endotherm it is understood that if one or more such endotherms are detected, the total heat of fusion is less than 1 J / g, preferably less than 0.5 J / g. By "crystallization exotherm" is meant an exothermic peak in the DSC due to an amorphous region that undergoes crystallization before polymer melting. By "vitreous transition temperature" Tg, we mean the inflection point of the gradual transition associated with the vitreous transition on a DSC trace heated to approximately 10 ° C / minute. An example of this is illustrated in Figure 1, which shows the inflection point of the vitreous transition occurring at approximately 58 ° C. By "apparent average temperature" of a pellet is meant the average temperature of the mass of the pellet or the average of the temperature at each site of the particle. By the term "thermal transfer coefficient" or "h" with respect to a solid surface or gas, to which a pellet is exposed, is meant k / b, where k is the thermal conductivity of the solid surface or gas, and b is the thickness. The values for k can be found, for example, in R.H. Perry et al., Chemical Engineers' Handbook, chapters 10-10 and 23 (McGraw-Hill Book Co. 4th edition) and R.L. Earl, Unit Operations: n Food Processing (Pergamon Press, Oxford 1966). For example, the heat transfer coefficient "h" for a steel strip of 1 mm thickness is, for example, about 1.5 joules / seconds cm2 ° C (or 2800 BTU / hour square foot ° F). However, the thermal transfer coefficient can vary between different steels. The heat transfer coefficient of natural convection nitrogen (as, for example, in a batch oven) can be, for example, from about 0.0005 to 0.002 joule / second cm2 ° C (or 1 to 4 BTU / hour square foot) F ), forced convection nitrogen (for example, as when a pellet falls under a force through a nitrogen column) can be, for example, 0.0025 to 0.05 joules / second cm2 ° C (or 5 to 100 BTU / hour) square foot ° F). The thermal transfer coefficient of a Teflon (MR) band of 1 mm can be, for example, from about 0.025 to 0.05 joules / second cm2 ° C (or 45 BTU / hour square feet ° F). In general, metals tend to have "h" in the range of about 0.10 to 40 joule / second cm2 ° C (or 175 to 67,000 BTU / hour square foot ° F). By the term "maximum" or "effective" (measured or effective T) maximum crystallization rate is understood the experimentally determined definition known in the art. The experimentally determined T: values can be found in the literature for a wide range of polyesters. For example, the maximum effective crystallization rate can be found experimentally as described in F. Van Antwerpen et al., J. Polym. Sci. Polym. Phys. Ed. , vol .. 10, p. 2423-2435 (1972); M: R: Tant and collaborators, Polym. Eng. And Sci ence, vol. 33, No. 17, p. 1152-1156 (1993); R. J. Phillips et al., Macromoi ecul es, vol. 22, No. 4, p. 1649-1655 (1989); S. Buchner et al., Polymer, vol. 30, p. 480-488 (1989). As indicated at the beginning, the calculated Tt, for purposes of this invention, is a reasonably accurate approximation of the effective T = for the polyester. The calculated T is defined herein as T = (Tg + Tm) / 2 or Tc = Tg + (1/2) (Tm - Tg). Polyesters are polymers generally comprised of one or more diacid or diester components and one or more diol components. Many polyesters can be formed into pellets by the process of this invention. The process of this invention is useful for most polyesters containing aromatic or aliphatic ring (eg, containing phenyl or cyclohexyl) that do not readily crystallize at room temperature. This could include, for example, poly (ethylene terephthalate) (PET), poly (ethylene naphthalate) (PEN), poly (butylene naphthalate) (PBN), poly (tri ethylene terephthalate) (3G-T) , and poly (trimethylene naphthalate) (3G-N), poly (cyclohexyl terephthalate) (PCT), and the like. In general, polyesters having a glass transition temperature, Tg, above about 25 ° C, and a melting temperature Tm in the range of about 200 to about 320 ° C are better suited for the process of this invention. The approximate Tg and Tm values for some useful polyesters are listed below in degrees Celsius.

PET 70 260 PEN 120 270 PBN 82 242 3G-T 35 227 The values for T3, Tc and Tm may vary somewhat, for example, with polymer morphology, thermal history, molecular weight and crystallinity. For example, a low DP PET (a DP of 10 to 20) typically has a Tt of about 250 ° C, a Tg of about 60 ° C, and a T of about 155 ° C. Suitable diacid or diester components for the polyesters to which this invention pertains, typically include alkyl dicarboxylic acids having from 4 to 36 carbon atoms, diesters of alkyl dicarboxylic acids having from 6 to 38 carbon atoms, aryl dicarboxylic acids which contain from 8 to 20 carbon atoms, diesters of aryl dicarboxylic acids containing from 10 to 22 carbon atoms, aryl-dicarboxylic acids substituted with alkyl, which contain from 9 to 22 carbon atoms, or diesters of aryl-dicarboxylic acids substituted with alkyl, which contain from 11 to 22 carbon atoms. Preferred alkyl dicarboxylic acids contain from 4 to 12 carbon atoms. Some representative examples of such alkyl dicarboxylic acids include glutaric acid, adipic acid, pimelic acid and the like. The preferred diesters of the alkyl dicarboxylic acids contain from 6 to 12 carbon atoms. A representative example of such a diester of an alkyl dicarboxylic acid is azelaic acid. Preferred aryl dicarboxylic acids contain from 8 to 16 carbon atoms. Some representative examples of the aryl dicarboxylic acids are terephthalic acid, iscphthalic acid and orthophthalic acid. The preferred diesters of the aryl dicarboxylic acids contain from 10 to 18 carbon atoms. Some representative examples of diesters of the aryl dicarboxylic acids include diethyl terephthalate, diethyl isophthalate, diethyl orthophthalate, dimethyl naphthalate, diethyl naphthalate, and the like. Preferred alkyl substituted aryl dicarboxylic acids contain from 9 to 16 carbon atoms and the preferred diesters of the alkyl-substituted aryl dicarboxylic acids contain from 11 to 15 carbon atoms. The diol component for the polyesters used in the invention appropriately includes glycols containing from 2 to 12 carbon atoms, glycol ethers containing from 4 to 12 carbon atoms and polyether glycols having the structural formula HO- (AO) PH , wherein A is an alkylene group containing 2 to 6 carbon atoms and wherein n is an integer from 2 to 400. In general, such polyether glycols will have a molecular weight of about 400 to 4000. Preferred glycols they usually contain from 2 to 8 carbon atoms with the preferred glycol ethers containing from 4 to 8 carbon atoms. Some representative examples of glycols that can be used as the diol component include ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 2,2-diethyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol. , 2-ethyl-2-butyl-l, 3-propanediol, 2-ethyl-2-isobutyl-l, 3-propanediol,, 3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1, 6 hexanediol, 2, 2, 4-trimethyl-1,6-hexanediol, 1,3-cyclohexodimethanol, 1,4-cyclohexanedimethanol, 2,2,4,4-tetramethyl-1,3-cyclobutanediol, and the like. The polyesters or oligomers of the present invention may be branched or unbranched, and may be homopolymers or copolymers. Particularly useful are "modified polyesters" which are defined as modified with up to 10% of a comonomer. Unless otherwise indicated, the term polyester means modified or unmodified polyester. Similarly, by the mention of a particular polyester, for example PET, is meant unmodified or modified PET. The comonomers may include diethylene glycol (DEG), triethylene glycol, 1,4-cyclohexane di-ethanol, isophthalic acid (IPA), 2,6-naphthalene dicarboxylic acid, adipic acid and mixtures thereof.

Preferred comonomers for poly (ethylene terephthalate) include 0-5 by weight of IPA and 0-3% by weight of DEG. In a prior integrated embodiment of this invention, the polyester prepolymer or oligomer constituting the pellets obtained by the present invention can optionally be polymerized from the monomers, oligomers or mixtures thereof. This optional polymerization step can be accomplished using known methods and apparatus, as will be readily appreciated by those skilled in the art. Polymerization of polyesters is well known in the art. Polyesters are often formed as a melt by combining a diacid or diester with a diol, to produce a monomer, and heating to polymerize the monomer. A preferred method of low molecular weight polyester polymerization is carried out in a pipe reactor. For details, see co-pending, commonly assigned application, W096 / 22318 published July 25, 1996, incorporated by reference herein. The polymerization is conducted to achieve a desired degree of polymerization. In general, the polyester used to make pellets according to this invention has a degree of polymerization in the range of about 2 to about 40. Per polymerization degree (DP) is meant the average number of repeating units in a chain polymer, and therefore may not necessarily be a whole number. For example, the repeating unit of poly (ethylene terephthalate) (PET) is O O 'r C-0-CH2CH2-0- - @ - ~ The DP of a polymer can be determined by Gel Permeation Chromatography using appropriate standards. The preferred degree of polymerization for this invention is influenced by the medium for the pellet formation that is chosen, and the anticipated use of the final pellets. In general, a DP of about 5 to 35 is preferred for PET, when the pelletizing means is a pelletizer, discussed below. The degree of polymerization is merely a way of expressing the molecular weight of a polyester. Another measure of molecular weight is the intrinsic viscosity (IV) of the polymer. For example, a polymer of poly (ethylene terephthalate) having a DP of 2 to 40 should have an IV which is - the range of 0.05 to about 0.4 dl / g when tested with a one-part solution in volume of trifluoroacetic acid and three parts by volume of methylene chloride. An IV can be determined according to the following example: 0.050 g of a polyester such as for example PET, is weighed in a clean dry flask, and 10 ml of solvent is added using a volumetric pipette. The bottle is closed (to prevent evaporation of the solvent) and stirred for 30 minutes or until the PET dissolves. The solution is emptied into a large tube of a Cannon-Fenske # 50 viscometer, which is then placed in a water bath at 25 ° C and allowed to equilibrate at that temperature. The times of fall between the upper and lower marks are then measured in triplicate, and must be in accordance within 0.4 seconds. A similar measurement is made in the viscometer for the solvent alone. The IV is then calculated by the equation: IV = ln [(solution time / solvent time) /0.5].

In one embodiment of this invention, a polyester polymer at or above its melting temperature is formed into pellets. The formation of pellets can be conducted by adapting various methods and apparatuses known in the art for the formation of pellets. This may include dripping (see Example 2), extrusion (see Example 3), pelletization, granulation (see for example, U.S. Patent No. 4,340,550 to Ho), spray atomization (see, eg, North American Rinehart patent). No. 4,165,420), and melt cutting, among others. A granulation / degassing device for polymers is available from Southwest Research Institute (Dallas, TX). Any method is appropriate as long as the polyester polymer can be formed in discrete portions at a temperature of or above its melting temperature, for example in the molten state. For polyesters, the melting temperature is usually in the range of about 200 ° C to 300 ° C, with polymers of lower DP that tend to have lower melting temperatures. For PET, for example, the melting temperature could usually be equal to or greater than about 250 ° C.

The formation of pellets, widely invited, is employed for the formation of particles in a preferred embodiment of the present invention. The pellet formation typically employs a cylindrical, rotating, outer container having a plurality of holes circumferentially spaced about its periphery. Inside the outer vessel is an internal coaxial cylindrical vessel having a measuring rod or channel. The plurality of holes on the outer container are positioned such that they will be aligned cyclically in the measuring bar or channel on the inner container, when the outer container is rotated. Typically, the molten polyester is transferred to the inner container of the pellet former and, under pressure, is supplied in uniform amounts, forming droplets or non-solidified pellets, as each of the plurality of orifices on the outer container align with the rod. measurement on the internal container. Pill formers are commercially available, for example, the ROTOFORMER® pellet former manufactured by Sandvik Process Systems (Totowa, NJ). For more details on the formation of polyester particles by the formation of pellets, see the co-pending application, commonly assigned and concurrently filed, US Serial No. 08 / 376,599, incorporated by reference herein. Preferably, the molten polyester transferred to the pelletizer or other pelletizing means is essentially amorphous. By essentially amorphous it is meant essentially non-crystalline, ie having less than about ten percent (10%), preferably less than five percent (5%), and more preferably less than one percent (1%) of crystalline content as defined by a DSC curve in which the difference in the melting value of the crystallization exotherm and the heat of fusion of the main melting endotherm is less than about 14 J / g, preferably less than 7 J / g, more preferably less than about 2 J / g. This is demonstrated in Figure 1, which shows a typical DSC curve for an amorphous polyester in which the heat of fusion of the crystallization exotherm (indicated by A) is approximately equal to the heat of fusion of the main fusion endotherm ( indicated by B), the difference between the two heats of fusion is less than about 4 J / g. The density can also characterize the crystallinity of various polyesters. For example, an essentially circular polyethylene terephthalate can, for example, be characterized by a density less than about 1.34 g / ml. Essentially amorphous polyester pellets are made in many commercial processes by rapidly quenching the molten polyester particles after formation, which quenching typically inhibits crystallization. In contrast, the essentially crystalline polyester is defined herein as possessing a crystallinity content greater than about 15%, preferably greater than 20%, and more preferably greater than 30%, corresponding to and respectively for PET, for example, at a density greater than about 1.36 g / cmJ, preferably greater than about 1.37 g / cm3, more preferably greater than 1.39 g / ml. Thus, the term essentially crystalline or crystalline, as used herein, will include what is commonly referred to as "semi-crystalline" as are most polyesters of interest. The amount of crystallinity can be determined by DSC. For example, essentially crystalline PET is characterized by a total heat of fusion, expressed in J / g, of at least about 20, more preferably about 35, when 140 J / g is used as the total heat of fusion of crystalline PET pure. Higher melting temperatures indicate more crystalline polymer. The percent crystallinity within a sample of a polyester material or pellet can be determined by comparing the heat of fusion (J / g) of the crystallites present with the heat of fusion of the "pure" crystalline polyester. After the formation of the droplets or the "melted pellets", the newly formed droplets are then crystallized. The crystallization step of the pellets is critical for the formation of robust, uniform, low molecular weight pellets suitable for a variety of uses including solid state polymerization. This crystallization involves temperature manipulations within time limits. Immediately after the polyester droplets are formed at the melting temperature, the droplets are rapidly subjected to thermal shock. Holding the newly formed polymer droplets at a temperature within a preselected or desired thermal range results in an immediate temperature gradient between the crystallization of the droplet or the pellet, initially at or near its melting temperature, and its environment surrounding. This must be done quickly, as shown herein, in order to obtain efficiently and / or optimally the desired crystal morphology. Of course, a temperature gradient also inherently occurs to some degree, depending on the circumstances, within the droplets or pellets that crystallize, since their apparent average temperature changes. Thermal shock is generally achieved through radiant heat, conduction and / or convection. Preferably, the heating is through the use, or mainly through the use of conductive or radiant heat, preferably radiant heat below 15 megahertz. The thermal shock is imposed in general, in order to ensure that the pellets rapidly reach a temperature in the temperature range as described above. Without wishing to be bound by theory, it is believed that this rapid change in particle temperature allows the particle to undergo a desired crystallization within the desired temperature range, instead of the particle suffering an undesirable amount and / or type of crystallization prior to reach this temperature range. This zone of temperature extends around what is termed as the calculated temperature of the maximum crystallization velocity. When an essentially crystalline polyester pellet is formed from an essentially amorphous melt, the process comprises forming molten droplets of a polyester oligomer at a temperature T-1 where Ti is at least the melting point T of the polyester oligomer , and wherein the polyester oligomer has a degree of polymerization (DP) of 2 to 40 and a glass transition temperature (Tg) above 25 ° C. Preferably, T: is between Tm and Tm + 30 ° C, more preferably between Tm and Tm + 10 ° C. The molten droplets are then crystallized by placing them in contact, for at least 3 seconds, with a solid surface which is at a temperature within the range of Tm? Na Tma as defined below, whereby the particles sustain a rapid change in temperature. the temperature towards said temperature, and remain at a temperature within said interval for a sufficient period of time. Preferably, the solid surface is metallic, since the metals will carry the droplets to the desired temperature more quickly. In the present invention, the T? R. = Tq + 10 ° C and T ~, ax = Tr + 0.5 (Tp - T-), where T = is defined as Tr = T + (1/2) (Tm - T3) or, equivalently, (1 / 2) (T-. + T :). However, if the solid surface has a thermal transfer coefficient (h3) that is below 1.5 joules / second cm ° C, then Tt.ln can be between 0 ° C and (Tg + 10 ° C), with the proviso that the apparent average temperature of the pellets remains above T-, n for at least 3 seconds after the pellets make contact with the solid surface and with the condition that the average apparent temperature of the pellets reaches Tl3X within 15 seconds, after the pellets come into contact with the solid surface. The exception for solid surfaces with relatively low thermal transfer coefficients, below 1.5 joules / second cm ° C, is more typically applicable to non-metallic surfaces. Preferably, Tmax = Tc + 0.3 (Tm-Tc and more preferably Tmax is approximately T- + 10 ° C. Preferably, at least for metal surfaces such as steel or aluminum, Tm? N = - 0.5 (T-Tg), and more preferably Tm? n Tc 0.3 (Tc-Tg), more preferably approximately T; - 10 ° C. In order to obtain rapid thermal transfer from a surface for newly formed molten droplets, it is preferred that the surface have a relatively high total thermal capacity.

Metals are particularly useful- .. for this purpose, especially metals with high coefficients of heat transfer. In this way, metals are the preferred materials for the surface, whose surface is moving preferably in a continuous process, as can be obtained with a conveyor belt. Preferably, the pellets are exposed to the solid surface in the stated temperature range for at least 3 seconds, more preferably at least 10 seconds, and still more preferably at least 20 seconds. The maximum time during which the pellets are exposed to the solid surface in the indicated temperature range is preferably 30 minutes, more preferably 10 minutes, and still more preferably 3 minutes. Longer times are no longer harmful, but may not be more economical. For example, in an integrated SSP plant for making high molecular weight PET, the pellets after being formed can be introduced into the SSP reactor within 10 minutes, after the pellets are formed. It is also possible to store the pellets for later use. For example, when PET pellets are made according to the present invention, an essentially crystalline PET pellet can be formed from an essentially amorphous melt of a PET oligomer at a temperature T: wherein Tx is at least 250 °. C, preferably between 260 ° C and 280 ° C, more preferably between 265 ° C and 275 ° C. The molten droplets are then crystallized by placing them in contact, for at least 3 seconds, with a solid surface which is at a temperature in the range of Tmin or Tmax where Tmin is preferably 80 ° C, more preferably 130 ° C, and still more preferably 150 ° C, except if the solid surface is non-metallic and has a heat transfer coefficient (hs) that is below 1.5 joules / second cm2 ° C, in which the TTLn event can be between 0 ° C and 80 ° C, depending on the process variables. Preferably, Tmax is 220 ° C, more preferably 200 ° C, and even more preferably 180 ° C. Alternatively, as indicated at the beginning, an essentially crystalline polyester particle can be formed starting with an essentially amorphous solid particle or pellet that has been previously made from a polymer melt. (By the term "solid" is meant a crystal or polymeric solid which is below its glass transition temperature). In this case, the invention comprises obtaining pellets of a solid, essentially amorphous polyester oligomer at a temperature t wherein the polyester oligomer has a degree of polymerization (DP) of 2 to 40, and a glass transition temperature (T3) above 25 ° C and, subsequently, the heat treatment of the pellets by contacting them with a gas at a temperature of at least Tm_- by at least about 0.5 seconds, where Tm? N is at least melting point of the polyester oligomer. Preferred gases for the transfer of broth include gases such as air, oxygen, carbon dioxide, nitrogen, argon, helium, and the like, etc. and mixtures thereof. Typically, the pellets can be exposed to radiant heat - or gases heated in an oven, either in a stationary system (in hes) or in a continuous system, for example, as when a conveyor belt carrying pellets is passed through of an oven. Alternatively, pellets can be dropped through a tower, for example, with a hot gas rising countercurrently. The pellets can be properly dropped on a hot or unheated surface, after falling. Allowing the pellets to fall into a liquid is less desirable, since the separation of the liquid is then required. When the pellets are heated, Tra? - is preferably between 270 ° C and 2000 ° C, more preferably between 300 and 1500 ° C, and even more preferably between 400 and 1000 ° C. Preferably, the exposure time, at the indicated temperature is between about 0.5 seconds and 2 minutes, more preferably between 1 and 60 seconds, depending on the temperature and the heating means. For example, drip heating of the droplets in a tower could preferably take place near the lower ends of the time slots, as compared to when the pellets are heated in an oven. The upper time limit may be especially preferred, or even necessary, when a hot gas is used which may otherwise melt the pellet if it is exposed too long. For example, when PET pellets are made according to the present invention, an essentially crystalline PET pellet can be taken from an essentially amorphous solid pellet, initially at a temperature below 90 ° C, preferably at least the temperature environment, and even more preferably below 70 ° C. The essentially amorphous pellets can be crystallized3 by contacting them, for at least 3 seconds, in an oven at a temperature of at least 250 ° C, preferably between 270 ° C and 1200 ° C, more preferably between 300 and 800 ° C. In more basic terms, with respect to the change effected within the particle, the present invention can be defined as a process for the crystallization of a low molecular weight polyester particle, comprising heating a pellet of solid polyester oligomer (crystalline), having the DP and Tg described above, from the T defined above, so that the average apparent temperature of the pellet is brought, within 15 seconds, to a temperature within a range extending from Tm? n to T a? where Tn? n = T - 0.5 (T; - Tg), Tax = Tc + 0.5 (Tm - T =), and T- = (1/2) (Tm + Tg), and, furthermore, after reaching said temperature, the pellet is kept within the range by minus 3 seconds Preferably, T ^, = T = - 0.3 (T, - Tß) and Tmax = T + 0.3 (Tm - T =). More preferably, Tm? N = T-30 ° C and T, ax = Tr + 30 ° C. Alternatively, a droplet of molten polyester oligomer can be cooled from Tc defined above, so that the average apparent temperature of the pellet is brought within 15 seconds, at a temperature within a range extending from T-n to Tmax, where Tm? N = T- -0.5 (T, - TJ, Ttax = T: + 0.5 (Tm - T :), and Tc = (1/2) (Tm + T) and, also, after reaching said temperature, the pellet is maintained within the range for at least 3 seconds Preferably, T ~, ln = Tc-0.3 (Tc-Tg) and Ttax = Tr + 0.3 (T-.-T), More preferably, Tm ? n = Tc -30 ° C and T ".ax = T- + 30 ° C. Thus, conceptually, if the pellet is cooled or heated, the thermal shock is analogous or similar, except that the shock or change temperature is negative when cooled, and positive when heated.The process parameters may vary, not only on the particular method and apparatus used, but may depend on the size or of the pellet, pellet geometry, molecular weight, heat transfer coefficient of the surface or gas with which the pellets come into contact, and the heat transfer area. In the present process, thermal shock can be achieved under a wide variety of circumstances and under a wide range of process limitations. For example, in the cooling of a melt, if the surface to which the pellets are exposed has a relatively low heat transfer coefficient, it may be necessary, in order that the pellets quickly reach the desired temperature, that the surface be significantly cooler than the desired temperature. For example, it may be necessary for the surface to be significantly cooler when the surface is a plastic such as poly (tetrafluoroethylene) Teflon ™: when the surface is steel. The surface of the molten polyester can be exposed to a combination of heat transfer materials, for example, a part of the surface can be exposed to a metal surface, and another part of the surface can be exposed to, for example, a gas . Similarly, when pellets are heated, part of the surface may be exposed to a metal surface, for example, in an oven. Liquids at the appropriate temperature may also be used, but these may be less preferred because contamination problems may occur, and due to the need to separate the liquid from the polyester. To determine the apparent average temperature of the pellets, the measurement of the average temperature can proceed as follows.

A sample of the pellets is quickly collected from the solid surface or gas, provided it is used for thermal shock of the pellets. The pellets are immediately placed in an insulated container, preferably evacuated. Preferably, the pellets almost fill the container. A thermocouple is inserted. The vessel is allowed to reach an equilibrium temperature and is recorded as the apparent average temperature. Alternatively, an apparent average temperature of the pellets that are processed can be obtained as follows. A sample of the pellets is collected. Immediately place the pellets in a pre-weighed amount of distilled water, at a known temperature, in a previously weighed insulated container. The total mass is reweighed. The equilibrium temperature is observed. The average apparent temperature of the pellets is calculated based on the following equation: < ? m,) x (cp) x (T. t »> (mp) X (3pP) X (Tp - T.) where mw is the mass of the water, cp "is the thermal capacity of the water, mP is the mass of the pellets, cpp is the thermal capacity of the pellets, Te is the equilibrium temperature, and T" is the initial temperature of the pellet, water, and X represents multiplication. This equation can be solved to determine Tp, the apparent temperature of the pellets. As will be appreciated by one of ordinary skill in the art, the apparent average temperature of the pellets, under various conditions, can be estimated with a reasonable degree of accuracy and accuracy based on the standard heat transfer equations. The person skilled in the art will be familiar with such calculations, including numerical and / or computer techniques, for improved efficiency and accuracy. For example, if someone knows the coefficient of thermal transfer of the environment and the conditions of the process, then an estimate of the change in the average apparent temperature of the particle can be obtained with time, from the equation: dTp «A (T.- • TP) Q - pipG dt dT - hA (T. "Tp) dt IUpCp dTp = kT.- • kTp dt where k = hA pipC TP t -Ln T.-Tp = kt Tp = Tp? (? _ Lct) + T.d-e "**) This equation indicates that if the thermal transfer constant, k, is known for a given system, as well as the initial temperature of the particle and the ambient temperature, then the average apparent temperature of the particle as a function of time can be calculated where mp is the mass of the pellet, cp is the thermal capacity of the pellet, t is the time, h is the coefficient of thermal transfer of the surface or of the gas to which the pellet is attached, Te is the temperature of the pellet surface or the gas to which the pellet is attached, and A is the area that is brought into contact or subjected to the heat source, whether it is a solid surface or a gas. , hemispherical dropped as a drop on a steel band can have a flat area A in contact with the band, whose area can be easily estimated as (p) (radius / 2)? At the same time, an average value A of a sample of pellets can be physical measured for use in the previous equations. These equations can be solved for T, the apparent average temperature of the pellet. As mentioned above, the thermal shock can be imposed so that the temperature gradient experienced by the pellets occurs in any direction, that is as a result of heating the cooling. However, it is preferable that the pellets are crystallized by cooling from the melt. This avoids the need to reheat cooled particles and is thus more energy efficient. Rapid crystallization may have the additional advantage of leading to the formation of crystals that are larger than those formed by conventional processes. See, for example, U.S. Patent No. 5,510,454, incorporated by reference herein in its entirety. While this invention is not compromised by any particular theory or explanation, it is believed that the crystallization carried out by the process of this invention is capable of providing crystalline nucleation and crystal growth rates that promote and enhance crystal formation more big. It has been surprisingly found that such pellets with larger crystals resist better, without adhering or agglomerating, the high temperatures associated with most SSP processes or even to allow higher SSP temperatures than those mentioned above a significant advantage, since SSP temperatures are a bottleneck in terms of time, compared to other polymerization processes. Without wishing to be compromised by theory, the crystallization of the pellet's outer layer, closer to the surface, may have more effect on the desirable characteristics of the pellets. Similarly, the present process is also capable of forming PET pellets, for example, which do not show a different prefusion endotherm when tested on a DSC. It is believed that prefusion endotherms are indicative of small and / or relatively imperfect crystallites. When prefusion endotherms are present the PET pellet will tend to adhere more rapidly to the other pellets when heated, usually at a temperature close to the prefusion endotherm, a very desirable trend in solid state polymerization, as discussed below. at the moment. As indicated at the beginning, Figure 2 shows a DSC curve for PET, not according to the invention, having a prefusion endotherm. Figures 3 and 4 are DSC curves for PET pellets, made by the process of this invention, which do not show endoterir.- ~ of pre-fusion. Also, as mentioned above, crystallized pellets are especially useful for solid state polymerization (SSP), in view of the amount and quality of crystallization within the pellets. The polymerization in solid state is well known to the expert. See, for example, F. Pilati in G. Alien, et al., Ed., Comprehensive Polymer Science, Vol. 5, p. 201-216 (Pergamon Press, Oxford 1989), which is incorporated by reference herein. The polymerization in solid state is particularly useful for the production of higher molecular weight PETs. In general, the PET particles are heated to a temperature below the melting point and anhydrous gas, usually nitrogen, usually concurrently in continuous operation, around and on the particles is passed. At high temperature, the transesterification and polycondensation reactions proceed and the gas can be used to remove volatile products (other similar methods, such as the use of a vacuum, can be used for this purpose), thereby promoting the high molecular weight of PET. In the past, a number of problems or difficulties have been associated with the solid state polymerization of PET. In particular, the particles to be polymerized usually had to undergo an annealing process, so that when they are heated during polymerization in the solid state, they do not undergo partial fusion and adhesion together. Alternatively, if the polymerization occurs at a relatively lower temperature to avoid adhesion, this could increase the polymerization time, since the reactions that drive the molecular weight rise proceed faster at higher temperatures. In any case, these difficulties or problems tend to make it more expensive to run the solid state polymerization process. Advantageously, the polyester pellets made by the process of the present invention can show superior crystalline morphology. For example, PET pellets can be made which can be directly polymerized (preferably without further crystallization or annealing) starting at higher temperatures, for example 230 ° C, preferably 240 ° C. This avoids the need for a prolonged annealing step, which lengthens the total process time. In addition, the particles produced according to the present invention can, in some at least at least, be more resistant to wear or friction, preventing the particles from wearing against each other or against the reactor in which they are contained. Thus, the use of the particles produced according to the present invention can result in an improved process for solid state polymerization. In any polymerization of low molecular weight polyester to higher molecular weight polyester, normal additives such as polymerization catalysts may be present. These may have been added when the low molecular weight polyester was formed. For example, a typical catalyst is Sb203, whose concentration in the present is given as the level of elemental antimony. Due to the initially higher polymerization temperatures in the solid state polymerization, using the low molecular weight crystalline polyester, as described herein, it may be possible to use lower catalyst levels when maintaining useful rates or polymerization rates. . Lower levels of catalyst may be advantageous when the polyester is designed for use in the manufacture of certain products, for example, when the polyester is intended for use in the manufacture of bottles, which will store beverages for human consumption. To provide an example of one embodiment of the present invention, PET pellets can be processed having an average crystallite size of about 9 nanometers or more, preferably 10 nanometers or more, more preferably about 12 nanometers or more, and especially preferably approximately 14 nanometers or more. The average size of the crystallites is measured by X-ray powder diffraction, wide angle, an exemplary method or procedure which is as follows: PET samples of uniform thickness for X-ray measurements are produced by criomolienda of the PET in a SPEX® Freezer / Mill (Metuchen, NJ) ba or liquid nitrogen for 30 seconds, and then compress the PET into disks approximately 1 mm thick and 32 mm in diameter. Due to the fragile nature of some of the PET discs, all discs are mounted on standard sample holders using the 3M Scotch® double-sided adhesive tape. Consequently, it is necessary to collect dust diffraction patterns from the PET discs (+ tape) and a tape control. While it is preferable that sample patterns are collected over the intercalo of 15-19 ° 2? (as shown in Figure 2), the patterns of the samples (+ tape) and a tape control can be collected over a range of 10-35 ° 2? in some cases, as was obtained for some of the samples (as shown in Figure 5). The diffraction data is collected using a diffractometer Phil1 ips automatic operating in a transmission mode (Cuka radiation, curved diffracted beam monocrometer, fixed gradual mode (0.05 ° / step), 65 seconds / step, 0 ° divisions, sample rotation). After subtraction, the powder diffraction pattern for the control tape is subtracted from each of the diffraction patterns showing more tape (sample + tape). The Lorentz polymerization corrections are applied for each powder pattern. To eliminate local backscattering in the region of 15 ° -19 ° 2? of each powder pattern, is defined a straight line that extends from 15.00 ° to 19.00 ° 2? and subtracts. This region of the diffraction pattern has been found to contain two crystalline reflections, at approximately 16.5 ° and 17.8 ° 2 ?, which have been defined as reflections (011) and (010), referred to by N.S. Murthy et al., In Polymer, vol. 31, p. 996-1002, incorporated by reference herein. Figures 5 and 6 show the diffraction patterns, corrected as detailed above, collected over the interval 10-35 ° and 15-19 ° of 2 ?, respectively. In addition to the Miller indices of the reflections of interest, the local "artificial" antecedent is shown between 15 ° and 19 ° 2 ?, marked "b", and described above. The region of 15-19 ° is then deconvolved into two overlapping Gaussian peaks corresponding to the two crystalline reflections, and the position, width and height of both peaks are extracted. An example of this deconvolution is shown in Figure 7. The apparent size of the crystallites for reflection (010) (here sometimes referred to simply as the apparent size of cpstalites), ACSo: ", is calculated from the position of the reflection and the full width at half height using the Scherrer equation, as for example described by LE Alexander, X-Ray Diffraction Methods in Polymer Science, p. 335 et seq. (John Wiley &Sons, New York, 1969): ACS 010 K? ßoioCOs? oio where ACSo :; is the average dimension of the crystal, K is assumed to be 1.0,? is of wavelength, ß is the full width at half height of the profile, in radians, and q has its normal meaning. In the following examples, certain analytical procedures are used. Apart from the X-ray diffraction, which is described in detail above, these procedures are described below. The references in the present to these types of analyzes, or their results, correspond to these ej emplares procedures.

Intrinsic Viscosity (IV) < • A solvent is made by mixing a volume of trifluoroacetic acid and three volumes. s of methylene chloride. The PET, in the amount of 0.050 g, is then weighed in a dry and clean bottle, and 10 ml of the solvent is added to it using a volumetric pipette. The bottle is closed (to prevent evaporation of the solvent) and stirred for 30 minutes or until the PET dissolves. The solution is emptied into the large tube of a Cannon-Fenske® # 50 viscometer, which is placed in a 25 ° C water bath and allowed to equilibrate at that temperature. The drip times between the upper and lower marks are then measured in triplicate, and must be in accordance with 0.4 seconds. A similar measurement is made in the viscometer for the solvent alone. The IV is then calculated by the equation: (solution time / solvent time IV = Ln 0.5 Gel Permeation Chromatography (GPC) The GPC was run on a Waters® 150C instrument ALC / GPC, using as a solvent hexafluoroisopropanol (HFIP) containing 1.3637 g of sodium trifluoroacetate per liter. The instrument was run in the usual manner, and standard calculations were performed to determine Mn (number average molecular weight) and Mw weight average molecular weight). The calibration of the instrument was performed using a PET sample with Mn of 22,800 and Mw of 50,100.

Fusion Point and Transition Temperature Vitrea The melting point was determined by Differential Scanning Calorimetry (DSC) and all samples were analyzed using a TA Instruments® DSC 910. The instrument was calibrated with indium, consistent with the system documentation. The samples were analyzed as they were received, no premolding was performed, using 5-10 mg ± 0.005 mg. The samples were sealed in aluminum drums and then heated from room temperature to 300 ° C to 10 ° C / minute in a nitrogen purged environment. The calculations of the glass transition temperature, the temperature of the melting point and the heat of fusion were made with the computer software (SOFTWARE) of the TA instrument. The peak DSC melting temperature, reported, is the corresponding peak temperature in the main fusion endotherm.

Thermomechanical Analysis A Mettler® TMA 40 Analyzer coupled to a TSC 10A controller was used for all samples. This instrument was calibrated for temperature using the standard operating procedure illustrated in the instruction manual at intervals of one month, or when spurious results were suspected. The samples did not have extra pretreatment in the TMA system that could alter the inherent raorfological history of the samples. The partial hemispherical particles were charged into the system in contact with the quartz fastener of the samples and a probe 3 mm in diameter such that the sample had the convex side up, with the probe in contact with the vertex of the hemisphere. Two temperature profiles were used to analyze the samples. The first is a high-speed exploration ratio of 10 ° C / minute from ambient temperature to fusion, and the second, to ensure a homogeneous heat environment, which is a speed of 1 ° C from 200 ° C to fusion.

EXAMPLES Example 1 - Thermal Shock Crystallization (TSC) of Solid Amorphous Particles PET was produced with an IV of 0.18 dl / g and COOH ends of 167.5 equivalent / 10 ° g, by a polymerization process in molten Ease and contained approximately 275 ppm of Sb as a catalyst. The melt was then extruded through a hole 1 mm in diameter to form droplets. The droplets fell through an air space of approximately 10 cm and were cooled with water to form clear amorphous particles. The particles were then shaped as pancakes, approximately 8 mm in diameter and 2.2 mm in thickness. The particles were crystallized one at a time in a Mettler TMA 40 apparatus coupled to a M ^ ttler 10A Thermal Controller. The individual particle was placed on top of the quartz sample holder at room temperature. The oven was preheated to 400 ° C, lowered onto the sample for 15 seconds, then removed allowing the particle to cool again to room temperature. After exposure to the oven the particle was opaque. The DSC analysis of the crystallized sample indicated that prefusion endotherms did not occur. The maximum melting temperature of 250.1 ° C. The ACSoio was 11.6 nanometers. The time it takes for a particle to crystallize depends on the molecular weight of the polymer, the size / geometry of the particle, the geometry of the heating source and the temperature of the heating source. To determine the effect of the temperature imposed on the crystallization rate, more of the above particles were crystallized in the TMA at various temperatures, then the same procedure as described above. The time to crystallize is the time elapsed from when the furnace was initially placed around the particle until the particle is observed to be completely opaque. The particle is observed through the upper opening in the furnace. The temperature of the furnace and the time required for the cri alization are given in Table I.

TABLE I Example 2 - Of the Melt on the Hot Plate PET was heated with an IV of 0.15 dl / g, and COOH ends of 188.2 equivalent / 10"g, which had been produced by a melt phase polymerization process, and which contained approximately 275 * ^ pm of Sb as a catalyst, in a Melt Indexer at 290 ° C until the polymer dripped out of the hole (1 mm in diameter) under its own weight.A warm plate covered with a steel plate of 1.9 cm in thickness was placed 15 to 25 cm under The temperature of the melt indexer was checked periodically by a thin wire thermocouple kept in intimate contact with the steel plate The polymer dripped onto the hot steel plate which was at 180 ° C. The crystallization was checked periodically by observing the clear amorphous drop that turned into an opaque solid.Once it was opaque the metal surface was stopped at an angle to the horizontal so that the particle could slide out and cool a at room temperature. The particles were shaped like pancakes, approximately 5.6 mm in diameter and 8.7 mm in thickness. The DSC analysis of the crystallized sample did not indicate prefusion endotherms. The maximum melting temperature was 250.3 ° C. The two particles formed by this method were placed one on top of the other in a sample holder, of quartz in a TMA and a load of 0.5 N was applied on them with the probe. The particles showed no signs of adhesion after being maintained for 30 minutes at 240 ° C under this load. PET was heated with an IV of 0.24 dl / g, and COOH ends of 27.8 equivalent / 106 g, which had been produced by a melt phase polymerization process, and which contained approximately 275 ppm of Sb as a catalyst, in a Melt Indexer at 290 ° C until the polymer dripped out of the hole (1 mm in diameter) under its own weight. A hot plate covered with a steel plate 1.9 cm thick was placed 15 to 25 cm below the hole of the melt indexer. The temperature was checked periodically by a thin wire thermocouple kept in intimate contact with the steel plate. The polymer dripped onto the hot steel plate which was at 180 ° C. The crystallization was checked periodically by observing the clear amorphous drop that turned into an opaque solid. Once it was opaque the metal surface was stopped at an angle to the horizontal so that the particle could slide out and cool to room temperature. The particles were shaped like pancakes, approximately 4.5 mm in diameter and 2.5 mm in thickness. The DSC analysis of the crystallized sample did not indicate prefusion endotherms. The maximum melting temperature was 258.7 ° C. The two particles formed by this method were placed one on top of the other in a sample holder, of quartz in a TMA and a load of 0.5 N was applied on them with the probe. The particles showed no signs of adhesion after being maintained for 30 minutes at 240 ° C under this load.

Example 3 - D ^ Cast on Turntable A PET with an IV of 0.21 dl / g and COOH ends of 141.0 equivalents / 10 °, which had been produced by a melt polymerization process and which contained approximately 275 ppm of Sb as a catalyst, was melted and processed at 255-280 ° C through a 16 mm twin screw extruder at 227 g / hour (0.5 pound / hour). The melt was extruded through a 1.0 mm die forming individual droplets that fell 1.3 cm through the air at room temperature on a hot rotary table. The rotary table provided precise regulation of surface temperature and residence time on the heated surface, continuous shaping of particles from the extruder. The device consisted of a rotary actuator driven by a gradual motion motor, a stainless steel rotating table in contact with a hot stationary plate. The surface temperature of the turntable was controlled through the manipulation of the temperature of the stationary plate. A calibration curve was generated for the measured, controlled temperature of the stationary plate versus the surface temperature of the rotary table, so that a thermocouple did not have to be coupled to the rotary table during crystallization. After approximately 300 ° of rotation on the turntable, the crystallized particles hit a block of Teflon® fluoropolymer which hit them off the turntable and into a collection vessel at room temperature. For the particles formed at surface temperature between 160-200 ° C there were no prefusion endotherms in the DSC traces. The processing conditions and particle analyzes are listed in Table II: TABLE II Comparative Example 4 To demonstrate the need to have the table temperature sufficiently hot so that the particles crystallize and are not partially quenched to the amorphous state, the same polymer and process was used as described in Example 3, except that the table was at lower temperatures. Four runs were made keeping the particles on the table for 28 seconds. The amount of amorphous material in a sample was determined from the DSC trace. The amorphous material that is easily crystallizable will crystallize during the DSC analysis and is observed as an exothermic peak. The amount of amorphous material is quantified by the size of the exothermic peak expressed in J / g. Run No. 1 in Table III shows that the lowest table temperature produces the largest amount of amorphous mate. Using this polymer and the processing conditions, the temperature of the table had to be at least 100 ° C to produce a good particle.

TABLE III Example 5 - Of the Melt on the Turntable A PET with an IV of 0.17 dl / g and COOH ends of 98.0 equivalents / 10 g, which had been produced by a melt phase polymerization process, and which contained approximately 275 ppm of Sb as a catalyst, melted and process through a twin screw extruder of 16 mm and dripped on a hot rotary table as described in Example 3. The processing conditions and particle analyzes are listed in Table IV: TABLE IV It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following:

Claims

1. A process for the crystallization of low molecular weight polyester pellets, having an average diameter of between 500 micrometers and 2 cm, characterized the process because it comprises the heating of essentially amorphous solid pellets of a polyester oligomer, which has a degree of polymerization (DP) from 2 to 40 and a glass transition temperature (Tg) above 25 ° C, from an initial temperature T, where T0 is below T, + 20 ° C, so that the average temperature Apparent pellets are brought, within 15 seconds, to a temperature within a range extending from T ^ "to Tmax where Tp? n = T- - 0.5 (T - Tg), Tmax = T + 0.5 (T - T), Tc is the calculated temperature of the maximum crystallization rate of the oligomer, defined as Tc = 0..5 (Trr + Tg), and Tm is the melting point of the oligomer, and, in addition, after reach said temperature, the pellets are kept within the range by a at least 3 seconds

2. A process for the crystallization of low molecular weight polyester pellets having an average diameter of between 500 microns and 2 cm, characterized the process because it comprises cooling the molten droplets of a polyester oligomer, having the DP and the Tg described above, from an initial temperature Ti, where Ti is at least the melting point Tm of the polyester oligomer, so that the apparent average temperature of the droplets or crystallized pellets is brought, within 15 seconds to a temperature within an interval which extends from T ^ to Tmax where Tmin = Tr - 0.5 (TC-Tg), Tmax = Tr + 0.5 (Trr - T), Tc is the calculated temperature of the maximum crystallization rate of the oligomer, defined as T.- = 0.5 (Tm + Tg), and Tm is the melting point of the oligomer, and, furthermore, after reaching said temperature, the pellets are maintained within the range for at least 3 seconds.

3. A process for the formation of essentially crystalline polyester pellets, having an average diameter between 500 micrometers and 2 cm, from an essentially amorphous melt, the process is characterized in that it comprises the following steps: a) the formation of molten droplets of a polyester oligomer at a temperature T: wherein Ti is at least the melting point Tt of the polyester oligomer, and wherein the polyester oligomer has a degree of polymerization (DP) of 2 to 40 and a glass transition temperature Tg above 25 ° C; b) the crystallization of the molten droplets by placing the droplets in contact, for at least 3 seconds, with a solid surface which is at a temperature within the range of Tm? n to T-, a, as defined more forward, whereby droplets or pellets that crystallize sustain a rapid change in temperature toward said temperature, and remain at a temperature within said range for a sufficient period of time; where T,? n = Tg + 10 ° C, Tnax = Tc + 0.5 (Tm -T-), where Tc is the calculated temperature of the maximum crystallization rate of the oligomer, defined as Tr + 0.5 (Tm - Tg) ); except that, if the solid surface has a heat transfer coefficient (hs) which is below 1.5 joules / second cm2 ° C, then Tm? n of the solid surface can be between 0 ° C and (Tg + 10 ° C) with the proviso that the average apparent temperature of the pellets remains above T, m for at least 3 seconds after the pellets make contact with the solid surface, and with the condition that the average apparent temperature of the pellets Pellets reach T ~ ax within 15 seconds, after the pellets come into contact with the solid surface.

4. A process for the formation of essentially crystalline polyester particles having an average diameter of between 500 micrometers and 2 cm, characterized the process because it comprises the steps: a) obtaining pellets of a solid, essentially amorphous polyester oligomer, to a temperature T where the polyester oligomer has a DP polymerization degree of 2 to 40, and a glass transition temperature (Tg) above 25 ° C; b) heat treatment of the pellets, putting them in contact with a gas at a temperature of at least Tm? n, for at least about 0.5 seconds, where Tm? n is at least the melting point of the polyester oligomer.

5. The process according to claim 1 or 2, characterized in that TmaR = 1 + 0.3 (T- - T :) and T -!., = Tc - 0.3 (TC - Tg).

6. The process according to claim 1 or 2, characterized in that Tmin = T = -30 ° C and T-.ax = Tr + 30 ° C.

7. The process according to claim 3, characterized in that Tmax = Tr + 0.3 (Tp- - T7) and Tmlr. = r - 0.5 (T: - Tg).

8. The process according to claim 3, characterized in that Tm? = Tc - 0.3 (Tc

9. The process according to claim 1, characterized in that the apparent average temperature of the pellets is brought to said temperature within at least 3 seconds.

10. The process according to claim 1, characterized in that the average apparent temperature of the pellets remains at said temperature for at least 60 seconds.

11. The process according to claim 1, characterized in that the polyester is PET and TTl3X is 220 ° C and Tro? R? It is 130 ° C.

12. The process according to claim 11, characterized in that Tmax is 200 ° C and Tm? R. It is 150 ° C.

13. The process according to claim 3, characterized in that Tmax is equal to Tc + 10 ° C.

14. The process according to claim 3, characterized in that Ti is between Tm and Tm + 30 ° C.

15. The process according to claim 3, characterized in that the solid surface is a metal.

16. The process according to claim 3, characterized in that the solid surface is a mobile surface for transporting the pellets.

17. The process according to claim 3, characterized in that the pellets are exposed to the solid surface in the indicated temperature range for at least 5 seconds.

18. The process according to claim 3, characterized in that the pellets are exposed to the solid surface in the indicated temperature range for not more than 30 minutes.

19. The process according to claim 3, characterized in that the pellets are exposed to the solid surface in the indicated temperature range, for a period of time between 10 seconds and 10 minutes.

20. The process according to claim 3, characterized in that the time in which the molten droplets are exposed to the outside, changes in temperature almost immediately after the molten droplets are formed.

21. The process according to claim 3, characterized in that the molten droplets are formed by a pelletizer.

22. The process according to claim 3, characterized in that the polyester is PET and essentially amorphous molten droplets, initially at a temperature between 250 ° C and 280 ° C, are crystallized by placing these in contact, for at least 3 seconds , with a solid surface that is at a temperature within the range of 80 ° C and 220 ° C.

23. The process according to claim 22, further characterized in that the essentially amorphous molten droplets are crystallized by placing them in contact, for at least 3 seconds, with a solid surface which is at a temperature within the range of 130 ° C and 200 ° C.

24. The process according to claim 4, characterized in that the droplets are treated with heat by contacting them with a gas at a temperature between T and 2000 ° C for at least about 0.5 seconds.

25. The process according to claim 24, characterized in that the exposure time at the indicated temperature is between approximately 0.5 seconds and 2 minutes.

26. The process according to claim 4, characterized in that the polyester is PET and the essentially amorphous solid pellets, initially at a temperature below 90 ° C, are exposed for at least about 0.5 seconds to a thermal environment containing a gaseous fluid at a temperature of at least 250 ° C.

27. The process according to claim 26, ccterized in that the essentially amorphous solid pellets, initially at a temperature between room temperature and 90 ° C, are exposed for at least about 0.5 seconds to a thermal environment containing a gaseous fluid at a temperature between 270 ° C and 1500 ° C.

28. The process according to claim 4, ccterized in that the pellets are formed by formation of pellets, formation of granules or cutting of the melt.

29. A process for the formation of high molecular weight polyester, by introducing the polyester pellets produced according to claims 1, 2, 3 or 4 into a reactor for subsequent polymerization.

30. The process according to claim 29, ccterized in that the reactor is a polymerization reactor e * - solid state essentially below the melting temperature of the material that is polymerized.

31. The process according to claim 29 or 30, ccterized in that the IV of the PET introduced into the reactor is below 0.3 and the IV of the high molecular weight polyester that is produced has an IV of at least 0.6.

32. The process according to claim 29 or 30, ccterized in that the pellets are introduced into the reactor without annealing steps.

33. The process according to claim 29 or 30, ccterized in that the polyester is PET.

34. A process for the crystallization of poly (ethylene terephthalate) pellets having an average diameter of 500 microns to 2 cm, ccterized the process because it comprises: the heating of a bright, essentially amorphous poly (terephthalate) having a degree of polymerization (DP) of 2 to 40, from an initial temperature below 90 ° C, to an apparent average temperature of approximately 120 ° C to approximately 210 ° C, within a period of time of 15 seconds and, in addition , after reaching said temperature, the pellets are kept within the interval for at least 3 seconds; or, alternatively, the molten pellets of the poly (ethylene terephthalate) oligomer, having the DP set forth above, are cooled from a temperature above the melting point of the oligomer, so that the apparent average temperature of the droplets or the droplets that crystallize are taken, within 15 seconds, at a temperature of 120 ° C to about 210 ° C and, after reaching said temperature, the pellets are kept within the range for at least 3 seconds.

35. The process according to claim 34, ccterized in that the temperature is from about 150 ° C to about 190 ° C.

36. The process according to claim 34, ccterized in that the apparent average temperature of the pellets is brought to said temperature within at least 3 seconds.